Recently, methods of producing a sugar liquid have been widely examined, which methods comprise a pretreatment of cellulose-containing biomass by an acid treatment, a hydrothermal treatment, an alkali treatment, or the like, followed by addition of filamentous fungus-derived cellulase for hydrolysis. However, as a drawback of such methods using filamentous fungus-derived cellulases, there is a problem in that the production cost of the sugar liquid increase due to a large amount of saccharification enzyme used and the a high price thereof.
As a technique to solve this problem, methods have been proposed, wherein filamentous fungus-derived cellulase used in cellulose hydrolysis is recovered and reused. Examples of what have been disclosed include a method comprising performing continuous solid-liquid separation by a spin filter, filtering a sugar liquid obtained by the separation through an ultrafiltration membrane, and recovering filamentous fungus-derived cellulase (Japanese Patent Application Laid-Open Publication No. 2006-87319); a method comprising adding a surfactant at the stage of enzymatic saccharification to inhibit adsorption of filamentous fungus-derived cellulase and thereby improve a recovery efficiency (Japanese Patent Application Laid-Open Publication No. 63-87994); a method comprising subjecting residues after enzymatic saccharification to electric treatment to recover filamentous fungus-derived cellulase component (Japanese Patent Application Laid-Open Publication No. 2008-206484); a method comprising performing a secondary hydrolysis of the saccharification residue to thereby increase the recovery amount of adsorbed enzyme (WO 2011/115039); and a method comprising performing a primary hydrolysis by first adding recovered cellulase and subsequently performing a secondary hydrolysis repeatedly by adding unused cellulase to increase the amount of enzyme recovered and the amount of sugars generated (WO 2011/115040). However, those methods have failed to reach fundamental solutions to the problem.
When hydrothermal treatment was employed as pretreatment when a sugar liquid was produced from cellulose-containing biomass, a large amount of hydrothermally-treated liquid was discharged when cellulose was hydrolyzed by using filamentous fungus cellulase after the hydrothermal treatment of the cellulose-containing biomass, which hydrothermally-treated liquid contained diluted oligosaccharides and enzymatic saccharification inhibitors such as furan-based compounds and aromatic compounds.
In view of this, it could be helpful to find a means of utilizing the hydrothermally-treated liquid which has been dealt with as a waste liquid at the time of the saccharification process of the cellulose-containing biomass and to reduce the amount of enzyme used in the hydrolysis of the cellulose-containing solid content.